I think it's more down to how you decide to play the game than diamond spending, finding a decent balance between houses, troops, tools and products replacing buildings once you have the required products for an entire age with another product building of the same dimensions saves spending diamonds or swapping the space created to housing, demolishing troop barracks when you have a decent number of troops and replacing it with houses etc. Sure diamond players will reap benefits in some ways, but you'd expect that in any game, but then again clever use of available resources and buildings and space are just as good as throwing diamonds about.
As yal-a-dar states most are just expansions in the research tree, some housing but as stated on the forum they aren't really worth having, I used the 500 diamonds to unlock 2 land expansions, purely so I can manufacture future needed goods now, and as such although I am a long way from the High Middle Age I am already producing needed products for that era. Once I have unlocked the necessary production buildings and so on I will have space for newer military and other useful buildings which in turn will allow quicker progress later on which I think will be a match for any diamond player, but time will tell.
Diamond players will I'm sure still have to spend time and forge points unlocking diamond specific research tech,and as the research tree shows thae path to these ends in a dead end anyway so no extra advantage there. that others won't have to put any points into at all. Ok they can buy an entire tech with diamonds in an instant but looking further into the tech trees some of those will be rather expensive (all depends on how many diamonds you get for your £,$ or Euro)